Code 172

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Oct 5, 2012 | 05:00 AM

pnp522

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Joined: Dec 2008

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From: Atascadero, CA

Code 172

Updating things on truck. Had to rebuild trans E4OD. While that was being done, the 5.0 was removed unplugged harnesses fuel lines and removed vacuum lines at the tree. Now back together, and have a 172 code that starts as soon as the thermostat opens. IAC was working fine till engine was installed again new one fixed that. Replaced HEGO sensor it had over 100,000 miles on it no effect. There are no vacuum leaks that I could find. There is an intermittent miss under partial throttle but that was there before but not as bad. Any ideas on where to start. My lovely Chilton manual tells you what the code means but no ideas on what to check other than take it to a shop.

Oct 6, 2012 | 07:52 AM

rla2005

Fleet Owner

Joined: Oct 2005

Posts: 20,789

Likes: 1,753

From: Kentucky

Quote:

Originally Posted by pnp522

Updating things on truck. Had to rebuild trans E4OD. While that was being done, the 5.0 was removed unplugged harnesses fuel lines and removed vacuum lines at the tree. Now back together, and have a 172 code that starts as soon as the thermostat opens. IAC was working fine till engine was installed again new one fixed that. Replaced HEGO sensor it had over 100,000 miles on it no effect. There are no vacuum leaks that I could find. There is an intermittent miss under partial throttle but that was there before but not as bad. Any ideas on where to start. My lovely Chilton manual tells you what the code means but no ideas on what to check other than take it to a shop.

That statement in bold could be a sign you do have a vacuum leak. There are several vacuum reservoirs that can be the root cause. No amount of spraying carb. cleaner or other tricks will reveal they are the cause. Testing each vacuum circuit with a hand held vacuum pump/gauge has always worked well for me.

What model/year truck do you have? If you have a Mass Airflow (MAF) style control system then you may need to check/clean/replace the MAF sensor. I would not recommend going that far until you know 100% you do not have a vacuum leak. Are there any other codes displayed during the KOEO tests or from CM?

Oct 6, 2012 | 08:14 AM

subford

Fleet Owner

Joined: Aug 2005

Posts: 23,667

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From: Easton,Ks

Code 172 means system indicates the engine is running lean.
As stated above it could be a vacuum leak.
Could also be low fuel pressure, bad wiring to the O2 sensor or a bad O2 sensor.